**Cr-Al-Zr-N**

A sputtered Cr-rich (CrAlZr)N coating with the composition of Cr38.8Al4.5V2N54.8 was shown to form a Cr2O3–rich tribolayer that decreases friction and wear [104].

#### 4.1.2. Coatings with the Addition of Refractory Metals

#### **Al-Cr-Mo-N**

Basic investigations of sputtered coatings showed that the addition of Mo significantly increases the hardness in a comparison, for example, of Cr20Al18.5 Mo7N54.5 at 37 GPa to unalloyed Al23Cr20N57 at 29 GPa [105]. Other (AlCrMo)N coatings (e.g., (Al50Cr37Mo13)N) were deposited by means of cathodic arc evaporation. The addition of Mo was shown to have a high potential to act as a triboactive coating when lubricated with lubricants containing sulphur [106]. (AlCrMo)N coatings have also been deposited by arc using Al70Cr30 and Cr70Mo30 cathodes [103]. The (AlCrMo)N coating had slightly higher hardness and a lower friction value and wear rate than the (Al70Cr30)N coating. The coating containing Mo showed a higher service life in a milling cutting test. A reason might be the formation of lubricious MoO*x* phases.

#### **Al-Cr-Hf-N**

AlCrN (Al/Cr = 3.2) and AlCrHfN coatings with an Al/Cr ratio > 3 and varying Hf content between 0 and 11.6 at.% have been synthetised by DC magnetron sputtering. The hardness was found to decrease slightly with an increasing Hf content, from 22 GPa to 19 GPa. The oxidation resistance of AlCrN increased by alloying it with a small amount of Hf of about 2 at.% (e.g., Al37Cr11Hf2N50); however, it decreases again at the highest Hf content [107].
